Due to increasingly strict governmental emission regulations, efforts to reduce emissions from IC engines have increased. These regulations have posed and will continue to pose particular problems for diesel engines, which while being very efficient, often produce very high emissions of NOx and particulate matter (“PM”). After treatment of diesel engine exhaust is often needed to help reduce diesel engine emissions, because diesel engines cannot be made sufficiently clean by modification of the engine design alone. This after treatment typically includes the use of a diesel particulate filter (“DPF”) and/or a NOx trap.
The DPF is typically a ceramic filter placed in the exhaust stream. The PM, which primarily consists of carbon particulates, collects in and on the DPF and clogs the filter, causing increased back pressure that reduces engine efficiency or makes the system inoperable. A NOx trap is placed in the exhaust stream and has an adsorbent that adsorbs the NOx from the exhaust stream. Similar to the DPF, when the adsorbent has reached its maximum adsorbing capacity, the NOx trap is essentially rendered useless in that no NOx is adsorbed. Both the DPF and NOx trap must be regenerated to maintain their effectiveness.
DPF regeneration usually occurs when the carbon particulates collected in the DPF are combusted when the DPF temperature rises past a minimum threshold. This increase in DPF temperature is typically a direct result from an increase in exhaust temperature when the engine is operated at high load. However, when engine operation does not provide engine exhaust temperatures high enough to regenerate the DPF, the exhaust stream or the DPF must be heated by other methods. For example, fuel may be combusted on the DPF or on a fuel combustor placed upstream of the DPF to heat the exhaust stream. However, a challenge for systems using this type of fuel injection and combustion is in obtaining a uniform concentration of fuel in the flow through the DPF, or through the upstream fuel combustor. Non-uniformity may lead to non-uniform cleaning, overheating, localized stresses, cracking, and other damage to the DPF or upstream fuel combustor.
A NOx trap typically contains an adsorbent-catalyst system that provides the dual functions of NOx trapping and NOx reduction. Usually one component of the NOx trap adsorbs NOx in the exhaust stream under oxidizing conditions, or conditions where the exhaust stream contains excess oxygen. This component is typically selected so that when the exhaust stream is made reducing, the NOx is released. The NOx trap also contains a NOx reduction catalyst upon which NOx reacts with a reducing agent under reducing conditions to form non-polluting N2. That is, when the exhaust stream is made reducing, the NOx is released and reacts with the reducing agent on the reduction catalyst to form N2.
As noted above, the NOx trap environment must be made reducing in order to convert the trapped NOx to N2. One way to produce this reducing environment is to modify the engine operation so that it operates in a rich mode. However, such a modification would require significant deviation from normal engine operation. For example, a diesel engine, which usually operates without a throttle on the air intake, would now require a throttle to drive the fuel air mixture into the rich regime. In addition, this would have to be done quickly and frequently, for example, about every 2 to 20 minutes. Similar, to the DPF regeneration mentioned above, fuel may be injected into the exhaust stream and combusted on the NOx trap or on an upstream fuel processor in order to both consume the oxygen and to produce the reducing environment, see for example Applicants Co-pending U.S. patent application Ser. Nos. 10/431,171 and 10/309,936, each of which is hereby incorporated by reference herein in their entirety. However, use of diesel fuel by direct injection into the exhaust stream is not very effective at exhaust stream temperatures ranging from 150° C. to 250° C., which covers a significant portion of the operating cycle of a diesel engine, including idle and low load.
Accordingly, improved systems and methods for treating fuel injected exhaust streams would be desirable. In addition, it would also be desirable to provide improved systems and methods for regenerating a DPF or NOx trap. In a like manner, it would be desirable to, provide systems and methods that could provide a substantially uniform fuel air mixture at the inlet of a DPF or a fuel combustor, and/or systems and methods capable of quickly heating a fuel combustor or fuel processor to a temperature within its operating range.